As it is known, many household and commercial appliances comprise a heating chamber. The working principle of the heating chamber depends on the kind of appliances. In some kind of appliances, like for example laundry drying machines (called also laundry driers), the heating chamber is structured to accommodate laundry to be dried, whereas in other kind of appliances, like for example microwave ovens, the heating chamber is structured to accommodate the food to be heated/cooked.
It is understood that in the present application with “commercial appliance” or “professional appliance” it is meant an appliance which is not designed to be used for “domestic” activities (even if theoretically it could be used also for domestic activities), but it is designed specifically to be used in commercial/professional activities such as, for example, restoration activities (restaurants, pubs, hotels), public service laundry (self-service laundry), or the like.
Some kind of known small commercial/professional cooking/heating appliances, generally called combined cooking appliances, comprises a number of different heating sources, such as microwaves generators, resistive heating means, and infrared radiation generating means. In use, the heating sources of the appliance are activated individually or in combination on the basis of the selected cooking/heating program, in order to perform quick cooking/heating of food products, especially sandwiches, toasts, hamburgers, met in general or the like.
Said commercial/professional cooking/heating appliances generally comprise a base member associated to a bottom heating surface designed to support food products to be cooked/heated, an upper member associated to a top heating surface and joined in an articulated manner to the base member in order to be tilted around an horizontal axis from an open position and a closed position, wherein the upper member is displaced towards the base member and the top heating surface comes to lie opposite to the bottom heating surface so as to enclose the food products therebetween.
The upper member is structured in order to close in onto the base member so as to form a cooking/heating cavity or chamber containing said heating surfaces. The base member comprises a microwave generator designed to irradiate the food products being enclosed between said heating surfaces, wherein the cooking/heating chamber defines a radiation shield or choke-frame designed to confine the microwaves radiation inside said cooking/heating chamber when the upper member is in the closed position.
To reach the fast cooking-time specifications, said combined cooking/heating appliances need to generate a high power density in the cooking/heating chamber. To this end, combined cooking/heating appliances are generally provided with two microwaves generators, i.e. two magnetrons which are generally placed in the base member below the food-support surface, and a high voltage control circuit which is configured to supply a high direct current (DC) voltage to the cathodes of said magnetrons.
Some kind of known high voltage control circuits of said combined cooking/heating appliances comprise two separate high voltage transformers and two rectifier circuit, each of which rectifies the alternate high voltage boosted by the respective high voltage transformer in order to supply the high direct voltage (or direct current D.C.) to the relative magnetron.
This solution has the drawbacks that said two high voltage transformers are weighty, bulky and heavily affect the overall cost of the appliance.
With the aim to overcome such problems, a solution is known wherein the high voltage control circuit comprises a single high voltage transformer which supplies both the magnetrons by using two relative half-wave voltage doubler circuits. The half-wave voltage doubler circuits are connected to the secondary high-voltage winding of the high voltage transformer, one in phase with respect to the other, in order that input terminals of both half-wave voltage doubler circuits have equal polarities during each half-period of the high-voltage.
In detail, half-wave voltage doubler circuits are connected in parallel to each other between a common terminal of the secondary high-voltage winding of the high voltage transformer and cathodes of the magnetrons and are configured to boosts and rectifies the high-voltage generated by the secondary high-voltage winding in order to provide a doubled high voltage to the magnetrons, respectively. The circuit structure and working of a half-wave voltage doubler circuit is disclosed, for example, in paragraph 7.6.1. of the book titled “THE COMPLETE MICROWAVE OVEN SERVICE HANDBOOK OPERATION MAINTENANCE TROUBLESHOOTING AND REPAIR” written by J. Carlton Gallawa.
In use, during the half-periods of the high alternating voltage, half-wave voltage doubler circuits operate “in phase” one to the other. More specifically, half-wave voltage doubler circuits are switched-on together during first half-periods of the high alternating voltage (for example during the positive half-waves), and they are switched-off together during second half-cycles (for example during the negative half-waves).
Thus, during the first half-cycles, the high voltage control circuit provides a maximum high power, which is substantially the sum of the in-phase magnetrons powers, whereas during the second half-cycles, the power provided to the heating chamber is zero as the half-wave voltage doubler circuits are switched-off.
However, supplying both magnetron powers simultaneously during the first half-cycles results in a too high power density, having very high undesirable power peaks inside of the cooking chamber.
Although this solution allows using a small transformer having less copper and laminated iron cores of smaller cross sectional area than the solution with two transformers, it has the drawback that the choke cover, in particular in case of few amount of food loaded in the cooking/heating chamber, can be subjected to electrical discharges due to said power peaks.
Indeed, the cooking/heating chamber of the combined cooking/heating appliances is quite small, thus the generated high power peaks produce localized high electric fields inside the chamber, in particular in correspondence of the choke cover. This may cause electrical discharges across the choke cover and high power losses due to eddy currents. Furthermore, the electrical discharges are further increased in the chamber by electrically conductive pollutants, e.g. food remains, water and may eventually lead to flashing.
Voltage doublers providing full-wave rectification for a single magnetron are also known from literature, but require many electronic components, thus they are not used in practice because too expensive.
The Applicant has conducted an in-depth study with the objective of providing a household or commercial heating appliances comprising a high voltage control circuit supplying high voltage to at least a couple of magnetrons, which is simple and cheap and is able to reduce the peaks in the power density and consequently the risk of electrical discharges in the choke cover, in the waveguides and in the heating chamber. It is thus the object of the present invention to provide a solution which allows achieving the objectives indicated above.